Tip-off apparatus for electron tubes



. J. L. GALLUP 2,934,860 TIP-OFF APPARATUS FORELECTRON TUBES May 3, 1960 2' Sheets-Sheet 1 Filed March 1, 1955 m z. fimmfy May 3,1960

FiledMarch 1, 1955 J. L. GALLUP TIP-OFF APPARATUS FOR ELECTRON TUBES 2 Sheets-Sheet 2 TIP- 3F APPARATUS FOR ELECTRON TUBES John L. Gallup, Bloomfield, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application March 1, 1955, Serial No. 491,459

1 Claim. (Cl. 49-2) This invention relates to an improved means for tipping-off the tubulation on electron tubes.

Electron tubes comprise a tubular envelope which contains the active elements or electrodes of the device. The envelope is essentially a gas tight container whereby the electrodes may be maintained in a vacuum or a gas having certain desired characteristics. To provide for the evacuation of the envelope during the fabrication of anelectron tube and the insertion of a particular gas if desired, there is an opening in the envelope to which is sealed in gas tight relation a tubulation sometimes referred to as an exhaust tubulation which may be made of glass. When the exhaust tubulation is sealed shut, the envelope is gas tight and the electron tube is ready for operation. The sealing of the exhaust tubulation is commonly referred to as tip-01f.

Where the exhaust tubulation is made of glass, tip-off has been accomplished in the past by heating a portion of the tubulation to soften it and subjecting the tubulation to axial tension forces thus stretching the softened glass at the heated portion of the tubulation sothat it constricts. The tubulation will finally collapse together at the heated portion due to the combination of the constriction produced by the tension forces and atmospheric pressure which tends to force the softened glass into the evacuated envelope. The heating is continued until the glass melts and the tubulation separates into two parts. Further heating of the exhaust tip or that portion of the tubulation which remains attached to the envelope tends to round oil and smooth out irregularities in the end surface of the tip.

It has been found that when the melting occurs at tipoif, a ball or bubble of molten glass will be forced inwardly into the tip by the atmospheric pressure. This ball of glass produces an uneven tension pattern in the tip. The tension results from the fact that the tip will cool from the outside inwardly, thus placing the inner surface of the tip in tension as the ball of glass cools and constricts away fromthe cooler and more rigid side walls of the tip. The uneven pattern results for the fact that the molten ball of glass tends' to separate from the inner surface of the tip at a very sharp angle thus producing a concentration. of the tension at'the point of separation. The combination of these factors tend to 1 make'the tip weak and in some cases will cause it to crack spontaneously thus allowing the entry of gasses into the envelope.

Therefore-it is an. object; of this invention to provide an improved means for tipping oii glass exhaust tubulationsp,

It is another object of this invention to provide a stronger tip for electron tubes.

It is a further object of this invention to provide a device for tipping oif the glass exhaust tubulation of an electron tube which will form the tip to a desired shape. Itisyet another object of this invention .to provide a tip for an electron tube having a contour designed for greater strength.

niteci States atent Briefly, this invention provides an improved apparatus for accomplishing the tip-off of a glass exhaust tubulation by heating the tubulation to a plastic state and bringing into contact therewith a plurality of opposed cutters. The invention further provides improved means for accomplishing the tip-oif of a glass exhaust tubulation comprising opposed cutters having a particular configuration for providing an improved tip-ofi. In addition, the invention provides an improved sealed exhaust tip for an electron discharge device.

The invention will be more completely understood when the following detailed description is read in conjunction with the appended two sheets of drawings, wherein: I

Figure 1 is a view in cross section ofa portion of an electron tube tipped off according to the prior art.

Figure 2 depicts a means for accomplishing a tip-off according to this invention.

Figure 3 is a view partially in cross section of an electron tube and a tipping tool according to this invention at the moment when the tip-off occurs.

Figure 4 is a view partially in cross-section of an electron tube tipped-oif according to this invention.

Figure 5 is a cross-sectionalview of another embodiment of the tipping tool according to this invention in its fully closed position. v

Figure 6 is a cross-sectional view taken along line 66 of the tipping tool shown in Figure 5.

Figure 7 is a cross-sectional view of yet another embodiment of the tipping tool according to this invention.

' Figured is a bottom view of one of the cutters of the tipping tool shown in Figure 7.

Figure 9 is a plan view of a complete tipping tool according to this invention.

Referring to Figure 1, an electron tube 1.0 is shown having a glass envelope 12 and a glass exhaust tubulation or tip 14 which was sealed by the tipping-off process of the prior art. A portion of the tubulation near the envelope 12 of the tube 10 was heated and while the heat was being applied, the tubulation was placed in tension. Thus, as the glass became more and more plastic under the influence of heat, the tubulation stretched at the point of heating, constricted, melted and was finally pulled in two.

. As the tubulation melted and was pulled in two, the melted glass closed the end of thetubulation'or tip 14 which remained attached to the envelope 12 and atmospheric pressure attempting to enter the evacuated envelope 10 forced a ball or bubble 16 of molten glass into the tip 14. At various points 18, where fusion between the molten glass andthe side wall 20 of. the tip does not occur due to the lower temperature of the side wall 20 at such points 18, the ball 16 of glass will become distinct from, though in contact with, such side wall 20. At these points 18, tension patterns will be set up in the side wall 20 of the tip 14 which will tend to cause cracks or .checks (not shown), extending radially outwardly in the sidewall 20 of the tip 14. This is due to the fact that the heat is dissipated from the tip 14 by conduction through the side wall 20 and radiation and convection from the outer surface of the tip 14. Since glass is a notoriously poor conductor ofheat, the outer surface of the tip '14 will be comparatively cool long before the innermost portion of the tip 14. Thus, as the innermost portion of the tip 14 cools and constricts, it will pull away from the cooler and more rigid outer portions of the tip 14" producing tension in the glass of the inner surface of'the tip 14. Any irregularities in the inner or tension surface of the tip 14 will tend to concentrate the tension at the pointof the irregularity. The

inwardly extending ball of glass 16 tends to increase the tension present due to its mass and high temperature and to produce sharp irregularities in the tension surface at the points 18 where it separates from the side wall 20 of the tip 14. a

The tension pattern thus produced will tend to cause the cracks or checks (not shown) previously mentioned aided by the poor strength of glass in tension. The cracks or checks will often develop slowly, due to residual tension and the fatigue characteristic of glass, sometimes penetrating the side wall 20 of the tip 14 destroying the vacuum tight seal. In most cases, the checks do not penetrate the side wall 20, but so weaken it that the slightest force on the tip 14 normal to its axis will cause it to crack or break off completely. It has been found that between and 40% of the electron tubes tipped off according to the prior art will have or will develop checked tips.

Figures 2 and 3 depict elements according to this in-.

vention for tipping oi the glass exhaust tubulation of an electron tube 10. Referring to Figure 2, the glass tubulation-22 of the electron tube 10 is inserted into a vacuum port 24 whereby the tube 10 may be evacuated. Heat is applied to the exhaust tubulation 22 at a point near the envelope 12 of the tube 10 as by the direction of gas flames 26 against the tubulation 22. The electron tube 10 should be supported by some means (tongs 28 for example) while the flames 26 are heating the tubulation 22 due to the fact that the tubulation 22' will be softened by the heat and would allow the tube 10 to fall if it were not independently supported. When the tubulation 22 has reached the plasticstate, opposed cut ters 30 (two for example) are brought into contact with the heated portion of the tubulation 22.

Referring to Figure 3, the cutters 30 collapse the tubulation 22 causing it to fuse shut, and sever it in two. It will beseen that the cutting edges 32 of theopposed cutters 30 have a configuration (semi-circular, according to the embodiment shown) for the purpose of imparting a desired shape to the tip 34 of the tubulation 22. As in this embodiment of the invention, the cutting edges 32 may be provided by forming depressions 36 and tapers one of a variety of materials. I- prefer to use metallic cutters due to the heat conducting properties of'metal in addition to the inherent strength and ruggedness of most metals. For example, the tungsten and tantalum car bides when bonded with cobalt or other metal of the iron group display good heat conducting properties, are capable of being formed to the shapes desired, and yet have sufiicient ruggedness to withstand extensive use at elevated temperatures.

The shape of the cutters 30 is determined by the desired contour of the inner surface 40 of the tip 34. I prefer a tip 34 having a hemispherically vaulted inner surface 40 due to the structural strength displayed by such contour. For this reason, I have used cutters 30 having concave depressions 36 formed therein which will comprise a hemispheric mold for the tip 34 when the cutters are closed. However, such cutters 30 are somewhat difiicult to fabricate. In addition, it has been found that softened glass is not exactly molded by bringing into contact therewith metallic molds. This is believed to be due to the cooling effect of the molds which tends to cause a more viscous skin or surface on the glass at the area of contact. Referring to Figure 3, it will be seen that the tip 34 produced is not in molded contact with the cutters 30. However, it has been found that by giving the cutters 3f the proper configuration, a desired contour may be produced in the inner surface 40 of the tip 34 although such contour may not be themolded image of the cutters.

For example, it has been found that trough shaped cutters comprising two co-axial, hollow half-cylinders, one having an outer surface matching the inner surface of the other so that one may telescope within the other, will produce a tip having an inner surface 40 that is substantially hemispheric. Referring to Figures 5 and 6, a tipping tool having hollow half-cylindrical cutters 44, 46 is shown in its closed position. The cutters 44, 46

comprise a first hollow half cylinder 44 having an inner 38 in the opposed ends of the cutters 30. The depressions 36 comprise a mold which aids the cutting edges 32 in imparting'the desired shape to the tip 34 and thus the cutters 30 are oriented with respect to the electron tube 10 so that the depressions 36 open toward the electron tube 10. The tapers 38 impart a sharpness or knife like characteristic to the cutting edges 32 to facilitate the accomplishment of the tip-off with a minimum of unevene ness and irregularity in the end surface 42 of the tip 34.

As soon as the tubulation 22 is severed, the tube 10 may be removed from the tip-off position without further heating. Due to the fact that the cutters 30 are unheated and may be'of heat conducting material, such as metal for example, the tip 34 is cooled before any appreciable amount' 'of glass may be rendered molten and forced inwardly of the tip 34 by atmospheric pressure.

Referring to Figure 4 which shows anelectron tube 19 tipped'ofr according to this invention, it will be seen that there is no ball of glass within the tip 34. Furthermore, it will be seen that the tip 34 hasa hemispherical or smoothly contoured, vaulted inner surface 40.

much greater tip strength due to the inherent stress distribution characteristic of such a surface in additionto the fact that the uneven strain patterns produced by the ball 16 of glass shown in Figure l are not present in the tip 34 shown in Figure 4. A further advantage of a tip 34 produced according to this invention is that it is less likely to be injurious to human hands due to the rounded or dome-shaped end surface 42. of the tip 34 as compared to the sharp spire-like end surface 42 of the tip 14'produced according tothe prior art. v

The cutters 30 used according to this invention may take a variety of shapes and may be composed of any It has been found that the vaulted inner surface 40 provides surface 48 of larger radius than that of the exhaust tubulation to be tipped off (twice as large, for example). A second hollow half-cylinder 46 co-axial with the first hollow half cylinder 44 having an inner surface 50 of approximately equal radius to the radius of the outer surface 52 of the first hollow half cylinder 44 is shown in engagement with the first half cylinder 44 such that an end of the first half cylinder 44 is inserted into an end of the second half cylinder 46. The opposed ends of the half cylinders 44, 46 which are brought into engagement have oppositely beveled edges 54 to provide each cutter 44, 46 with a knife like cutting action. That is, the inner surface 48 of the first half cylinder 44 is shorter than its outer surface 52 whereas the outer surface 56 of the second half cylinder .46 is shorter than its inner surface 50. The outer surface 52 of the first half cylinder 44 and the inner surface 50 of the second half cylinder 46 are brought into; sliding contact with each other thus providing positive cutting action.

The cutters are oriented "with respectto the electron tube 10 and exhaust tubulation (not'shown in Figures 5 or 6) so that their concave 'inner surfaces 48 and 59 are presented toward the electron tube and their 'axis is perpendicular to the axis-of the exhaustv tubulation. Thus, the semi-circular cutting edges 5410f the cutters 4 4, 46, will extend transversely across the exhaust tubulation'22 and will arch away from the electron tube 10 to impart curvature to the tip 34. In addition, when the cutters 44, 46 are brought into contact with the tubulation at the beginning of theircutting action, they will tend to deform portions of the tip adjacent the cutters but not in contact herewith. This deformation will con- 'stitute a curvature of the side walls of the tip 3-4 as they are compressed, which when combined with the curvature heretofore mentioned will impart to the inner surface 40 er the tip 34 a vaulted or substantially hemispheric con- 0111.

The telescoping or overlapping of the half cylindrical cutters 44, 46 provides a positive cutting action which is not easily obtained with hemispheric mold type of cutters 30. It will be apparent that the quarter spherical depression 36 of the cutters 30 shown in Figures 2 and 3 could not be adapted to telescope or overlap to any useful extent without imparting to the cutters 30 some rotary motion in addition to the rectilinear motion 58 as indicated. The telescoping action of the half cylindrical cutters 44, 46 insures a clean cut-off of the tubulation without the production of a fin or ridge of compressed glass on the end surface 42 of the tip 34 at the point of contact of the cutters 44, 46.

Referring to Figures 7 and 8, additional advantages may be obtained by providing the half cylindrical cutters 44, 46 with re-entrant cutting edges 60. According to the embodiment shown, the opposed ends of the half cylindrical cutters 44, 46 are each shortened intermediate their side edges 62 in planes at an angle to their axes, forming an arcuate and re-entrant cutting edge therein. When these cutters 44, 46 are advanced toward each other, the portions of their cutting edges 60 adjacent their side edges 62 will come into engagement first and their cutting edges 60 will form an aperture which will constrict from all sides as the advance of the cutters 4-4, 46 is continued. This cutting action will impart additional curvature to the tip 34 produced which will more nearly approximate the desired hemispheric shape.

A lip or flange 64 may be provided on each side edge 62 of the larger half cylinder 46 adjacent the cutting edge 60 thereof to contact the side edges 62 of the smaller half cylinder and insure the close engagement of the two cutters 44, 46.

The means for bringing cutters, according to this invention, into engagement may also take a variety of forms. In a successfully tested device, the hemispherical mold type of cutters 30 such as are shown in Figures 2 and 3 having a radius of approximately 2 centimeters, were attached to the jaws 66 of a plier-like device 68 as shown in Figure 9. An electron tube 10 having a glass tubulation 22 of approximately 1.5 centimeters radius was evacuated and a portion of the tubulation 22 heated. The electron tube 10 was supported while the tubulation 22 was heated as by a pair of tongs 28. The pliers 68 having the shaped cutters 30 were then closed on the heated portion of the tubulation 22 to tip-off the tube 10. Tips 34 produced by this method were found to be free of the ball or bubble of glass and the cracks or checks produced thereby and to have a vaulted and substantially hemispheric inner surface 40.

In a special testing device, the tips 34 produced as described according to this invention where found to withstand thirty or more pounds of force applied normally to the axis of the tip whereas tips made according to the prior art could only withstand forces of approximately twenty-five pounds if unchecked and eight pounds if checked.

In addition to the advantage of producing stronger tips, another advantage of this invention is that the tipotf may be made in much less time than was necessary according to the prior art. This is due to the fact that the heated portion of the glass tubulation does not have to be raised to its melting temperature according to this invention whereas it had to be actually melted in two according to the prior art. This fact has two advantageous aspects, first in that the actual heating time is less and second in that the flames of the tipping torches or other heating means may be made hotter for even greater speed without any adverse effects, whereas if the flames are made hotter in the tipping-off process of the prior art in order to speed up: the process, more checked tips will result due to the relativelylarger ball of inwardly extending glass produced thereby.

It is apparent that an improved glass tip for an electron tube is herein desoribed. In addition, an improved method of and means for tipping-oil the glass tubulation of an electron tube is herein provided which is inherently suited for use with automatic machinery for the fabrication of electron tubes Where the tip-off must be made rapidly in order to keep up with other automatic operations.

What is claimed is:

Apparatus for tipping off an evacuated electron tube having a pre-heated glass exhaust tubulation comprising two hollow half-cylindrical cutters, one end of each of said cutters being shortened intermediate the side edges thereof whereby an arcuate and re-entrant cutting edge is provided, one of said cutters having an outer surface and the other of said cutters having an inner surface of substantially equal radius, and means attached to said cutters for positioning said cutters in spaced co-axial relationship, whereby said cutting edges of said cutters are opposed to each other, and for moving said cutters axially toward each other and into engagement, whereby said one of said cutters is inserted into said other of said cutters.

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